Let's see what the rules have to say on this issue:
Metagaming: This is when characters act on information
that they don’t have access to, but which their players know
from the real world. Metagaming comes into play when
players fail to maintain a divide between in-character
knowledge and out-of-character knowledge. That could
include anything from uncannily accurate in-character
predictions from a player who’s already read the adventure,
players recognizing monsters when their characters
wouldn’t, low-Intelligence characters accessing well educated players’ knowledge and talents, etc. (Pathfinder RPG GameMastery Guide p.9)
One particularly sticky area of metagaming has
nothing to do with game mechanics, but rather real-world knowledge and intelligence. Sometimes the player
who’s a genius at solving puzzles and riddles wants to
play a dumb brute of a swordsman. This is great—so long
as his character isn’t still solving all the puzzles. In this
situation (or the reverse, where the player who’s terrible at
puzzles has an Intelligence score of 22), let all the players
work together to solve the puzzle, but use skill checks
and Intelligence checks to offer hints or determine who
actually comes up with the solution. Similarly, don’t fall
into the trap of letting a player’s knowledge base inform
the character’s beyond what’s reasonable. Just because
your player knows how to make gunpowder out of bat
guano doesn’t mean his uneducated halfling cleric does. (Pathfinder RPG GameMastery Guide p.9)
Though the above quotes are under the topic of metagaming, they clearly show to what degree that ability scores matter when role-playing a character.
James Jacobs (Paizo Creative Director) advises (in support of the rules stated above) on the Paizo forums
It's easier for the GM to play a high Int NPC, since he can use metagame info to fake the high intellect. For a PC, I sometimes do the same, but I also tend to put a few extra skill points in knowledge skills to help set up the fact that my character's a smartie. I also try to use more metagame knowledge as well, subject to GM approval. Patrick plays a super smart wizard in one of my games, and he plays his character as slightly mad—other folks are just to stupid to understand the reasons he does what he does. He sometimes laces made-up words into his character's dialogue and then defines them for the other players, simulating a larger vocabulary than other characters (and other players).
Low mental ability scores generally indicate some sort of madness or brain damage or personality flaw.
Low INT: Play the character by making dumb choices or having trouble reading or the like. Avoid putting ANY ranks into Int-based skills.
Low WIS: Play the character as super impulsive, little common sense, and kind of oblivious to things. Get distracted a lot.
Low CHA: Play the character as a jerk, or alternately as someone who just doesn't have much personality at all. A character who simply doesn't say much or hangs back during non-combat encounters. Frankly... this type of character, the one who avoids actual roleplaying or is antagonistic, is my LEAST favorite type of character to play, so my characters tend to always have above average Charisma scores if possible.
Since you are playing Pathfinder, explain to the player that in the Pathfinder rules, ability scores are directly tied to how you play your character. In addition to the Pathfinder RPG GameMastery Guide quotes above, have him read the Ability scores section of the Core Rulebook (p.15-17). This page has the same information, except where noted. Ability scores inform the player where the character stands in the game world. Let's take a look at their descriptions.
Strength (Str): Strength measures muscle and physical power.
Dexterity (Dex): Dexterity measures agility, reflexes, and balance.
Constitution (Con): Constitution represents your character’s health and stamina.
Intelligence (Int): Intelligence determines how well your character learns and reasons.
Wisdom (Wis): Wisdom describes a character’s willpower, common sense, awareness, and intuition.
Charisma (Cha): Charisma measures a character’s personality, personal magnetism, ability to lead, and appearance.
Why is it important to try to role-play according to your character's ability scores, besides it being an integral part of the rules? The 3.5 Dungeon Master's Guide helps with this stating:
The most important purpose of a campaign is to make the players feel that their characters live in a real world. This appearance of realism, also called verisimilitude, is important because it allows the players to stop feeling like they’re playing a game and start feeling more like they’re playing roles. (DMG 3.5 p.129)
The rules for ability scores are meant to help facilitate this very thing. Just as numbers help us make sense of reality, they also help us make sense of a fantasy world.
As an example, lets pit a human with a Strength of 20 vs a human with a strength 5. By the rules the character with the Strength of 20 can lift 400 pounds over his head. The character with a Strength of 5 can only lift a 100 pounds over his head. No matter how a player wants to role-play, this is a fact.
I once had a player, that played a character with a Wisdom of 20 extremely impulsive and rushed into deadly situations. After a few times of asking if he really wanted to do such things (to which he replied "Yes"). I asked him did he really think a character with a Wisdom of 20 would do these things? He said, "No, it's kinda the opposite of being wise, huh?". Afterwards, he started playing the character in a wiser fashion, and he was happy. When a character has a high Intelligence, Wisdom, or both, I let the group put their heads together (if they want to) to help make the character's choices more believable (like what the GameMastery Guide states above).
I shared all the above with one of my groups Saturday night. The result...it helped each member become more involved in the role-play of their characters. Yes!
Obviously a player is still free to play how he wants. Let him know that if he plays his 5 intelligence character to smart he's metagaming, and explain why it's harmful to the game (i.e. it disparages belief in the fantasy world you've created).
Does this mean that a 5 intelligence character can never have a good idea, obviously not. Have him roll intelligence checks. If he's succeeds at a notable amount, which is doubtful, he can keep up the ruse a bit longer. He could also make bluff checks to appear more intelligent than he really is.
If he's playing with high scores as if they're low (e.g. a wizard with a 20 intelligence playing as if it was 5) have him make bluff checks as he is trying to hide his true intellect or clarify if his character is insane.
The Pathfinder rules are on your side, but it takes players and a GM to play. Most players will cut the GM some slack if he can give good reasons for his decisions, seeing as he does most of the work.